The present invention relates to the vacuum pump arts. It finds particular application in a helical screw rotor vacuum pump.
Screw vacuum pumps include two pairs of helical rotors attached to shafts which are driven at high speed by an electric motor positioned below the shafts. The rotors have a plurality of teeth on their edge or arrayed on one or both of their faces and, in use, the teeth rotate within a pumping chamber and urge molecules of gas being pumped through the pumping chamber.
A gearbox is usually positioned at the driven end of each shaft. The gearbox contains the shaft ends, bearings within which the shaft rotates, any timing gears and the motor positioned about the driven shaft.
Oils and/or greases associated with lubrication of the gearbox need to be contained and isolated within the gearbox. This is to ensure cleanliness and prevent non-contamination of the gases being pumped in the pumping chamber and to avoid the possibility of transfer of such contamination back into the enclosure being evacuated.
The conventional screw vacuum pump has working rooms for compressing fluid (gas) by decreasing its volume and working rooms which have no compression action on the fluid, but has merely a fluid feeding action. Therefore, in the conventional screw vacuum pump, the pressure rises up locally (at the portion which has the compression action), and this local rise-up of the pressure causes an abnormal temperature increase at parts of the rotors and the casing of the vacuum pump. That is, the temperature at the discharge side at which the working room reduces its volume and thus compresses the gas tends to abnormally rise up. As a result, the members constituting the screw vacuum pump are un-uniformly thermally expanded due to the local temperature increase, and thus the dimensional precision of the gap between the casing and the rotors and the engaging portion's gap between the male rotor and the female rotor cannot be set to a high value.
In some prior art screw vacuum pumps, pressure adjustment devices are provided on the lower surface of the casing and in the axial direction of the rotors in order to prevent excessive rise-up of the pressure of the working rooms and thus prevent the abnormal temperature rise-up of the vacuum pump when the vacuum pump works in a state where the suck-in pressure is substantially equal to the atmospheric pressure.
Minimizing power consumption in the pump is an on-going challenge. Existing pump systems include suction sections at the ends of the rotors adjacent the closed end plates. The roots portions are provided at each of the both ends of the screw gear portions; that is, they are provided at both the suck-in side and the discharge port. A roots stage is needed adjacent the end plates. Including the suction sections at the ends of the rotor results in a less efficient compression and a smaller reduction in temperature. The roots portions of the existing pumps are difficult to machine and do not result in an appreciably larger volume of gas being trapped and accordingly result in less efficient compression.
Accordingly, it is considered desirable to develop an improvement to the power consumption of the pump condition which would reduce power needs at high pressures and reduce rotor sizes, which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.